Monday, November 22, 2010

From The Horses Mouth... Hence Surly's Blog

Surly Spew
#14 - Pugsley
What Makes Pugsley So Weird?

It all starts with the tires. Big, fat tires, particularly run at low pressure so they flatten out a bit, provide more ground contact and better traction than any standard tire. It allows the tires to ride over the top of soft stuff. It’s the same way snowshoes work: spread the load out over more surface area and you stay mostly on top of the snow instead of sinking way down into it. It’s this theory that drove the design of the Pugsley: a bike you can actually ride not just off road but where there is no road or trail, without tearing up the earth, without sinking into the snow, sand or loamy soil, and without having to get off and walk.

To accomplish this, you need big tires (and with really big tires, it helps to have really big rims too). Standard frames simply are not designed for really big tires (anything 3.0” or larger, and most frames don’t even accept tires this big) because most people, for most riding, don’t need the float and don’t want the weight of big tires. But in certain situations, big tires are the best way to get around. So to use really big tires, you have to make a frame’s seat- and chain stays wider. And to keep the chain stays as short as possible (so it handles like a bike instead of a truck; in other words, the Pugsley is responsive and maneuverable despite it’s bulky look), and as well to help gain chain clearance (more on this in a second), you also need to use a wide bottom bracket. We went with the widest standard BB out there, a 100mm shell (most bike frames use either a 68 or 73mm shell), because our Endomorph tires are a whopping 3.7”.

Now then, you’ve made a frame capable of running really big tires, but even with the wide BB, you’ll still have chain clearance issues. Our tires are so wide that in terms of chain line they overlap the innermost gears. In other words, when running in the easy gears (closest to the frame), your chain will hit your tire. Not good.

So what to do? Well, we curved our seat and chain stays right of center, and the wheels for the Pugsley need to be built offset to match (in other words they’re dished, but the opposite direction from normal). This gave us all the chain line we needed. Rear end solved.

Which brings us to the fork. On your average fork, the crown is either as wide as, or narrower than, the hub. In order to allow not just tire clearance, but snow and mud clearance too, the fork for a bike such as the Pugsley would have to be much wider at the crown than at the hub, and front hubs are narrower than rears which amplifies this difference. Making a fork with a wide crown with legs that pinch down at he hub works o.k., but it creates problems getting the wheel in and out, and as the Pugsley is designed and built to be a true adventure bike, it seemed sensible to us to design the fork spacing so it would accept a rear hub (135mm mountain hub standard), thereby achieving good clearance without the pinched look while allowing the front and rear wheels to be interchangeable.

This may not seem like such a big deal, but messy conditions can wreak havoc with your parts, particularly in extreme cold. Freewheels or freehubs that work fine the rest of the time, for example, can fail and just spin with the drive pawls open when it’s cold enough to freeze the grease inside, and when you’re in the middle of the arctic at 40 degrees below zero Fahrenheit, this sort of failure is not just annoying but quite dangerous. Having a spare operational wheel can mean the difference between getting out under your own steam and leaving on a Life Flight helicopter.

Now, since the frame is offset to allow chain clearance, the wheels need to be built offset of center too (note: we offer several varieties of Large Marge rims, and some are asymmetrically drilled specifically for use on the Pugsley). We curved one of the fork blades to accommodate the wider hub and offset rim. Problem solved. It may look cartoonish, but we didn't make it that way to draw stares or to be goofy. There is a method to our madness. Function first.